In my limited understanding, Core's utxoset design results in a lot of linear processes, whereas the libbitcoin model (on which Rearden based this I think) is much more easily parallelized.

I think when downloading blocks, you can check all sorts of things before you check the truly linear thing (do these inputs exist and have they been spent before). Core doesn't necessarily do all the optimization in can here because it is based on the design of Satoshi's original client.